(these notes are being posted in two parts to make the length more manageable, part 1 is here)

Continuing from where we left off, about topics discussed at the PipeWire hackfest in Nice…

DSP features

We discussed a number of features related to digital signal processing blocks which are typically realised on specialised hardware (often a DSP core that can directly interface with physical audio inputs and outputs on your laptop/phone/…).

There is currently no standard way for the firmware running on these DSPs to signal what features can be realised directly on DSP. We also would want to allow such features, if exposed from PipeWire, to be realisable on CPU.

Now we do have a way to hide away signal processing in a specific node, which is the filter-graph parameter on the audioconvert node that wraps all audio nodes.

We could extend this mechanism to allow the internal node (say the ALSA node implementation), to expose what filtering it can perform “in hardware” (i.e. the software running on DSP). This would allow the audioconvert to delegate some or all processing to the internal node, with fallbacks available on the CPU.

We would need a number of pieces to do this, including:

• Some standard definition of filters and associated parameters, so different implementations could have a standard “API” to express any given filter.

• The DSP block would need to expose what features it has and how they might be used. We could imagine extending the ALSA UCM configuration to do that.

• The audioconvert node would need to have a way to push down filter-graph params to the internal node, and negotiate what work it is doing vs. what is being delegated

This is a non-trivial effort, but gives us some sketch of what might be possible.

More DSP features

In addition to standard filters, we spoke about two topics that have come up commonly in the past.

The first is some way to expose the processing graph in the DSP, so PipeWire and other userspace daemons have a better view of what is happening on the DSP. With the ability to push dynamic topologies to DSP, there was some renewed interest in exposing and using the ASoC DAPM widget graph. As always, the devil is in the details.

The second thing that came up is speaker calibration. There is a lot of processing and tuning that goes into driving speakers on modern devices as much as possible without destroying them. Some of these are one-time parameters decided at product design time, and some of these translate to runtime parameters based on voltage and current feedback from the speaker amplifier.

For some systems (like Qualcomm platforms), speaker calibration might be run on each system start to perform dynamic tuning. We had some discussion of how this might tie in with the rest of the system for both determining the parameters (separate startup daemon vs. in-process initialisation), as well as uploading parameters to the speaker (some ALSA UCM extensions to load parameters on PCM open but before start, or preloading parameters into ALSA kernel controls and having the driver feed them in at the right point).

Volume limits

A way to set a limit on the maximum volume for a given device has been a common user request ([1] [2]). We discussed the possibility of creating a per-route property (with a fallback to the node, if there are no routes), which WirePlumber could manage to provide users a simple interface to control.

Since the hackfest, Wim has already done some work on this, and we need to bubble this up as a more user-accessible setting.

Performance

A number of performance-related topics were discussed.

The first was an option of a combined DSP mode, where instead of one port per channel, a node would expose one port for all the channels of the stream (but continue to run in the configured “DSP” format/rate). This would improve stream performance for non-JACK-like use-cases, especially in resource-constrained environments.

On the WirePlumber side, there was a discussion about using LuaJIT instead of standard Lua. There are some compatibility issues to be determined there (such as language version supported, etc.), but there might be some quick performance wins to be made if this is feasible.

There is a plan to move some of the WirePlumber core to Rust, and that might be a good time to also port over some of the more standard functionality that tends not to change from Lua to Rust (though that could happen in a Lua->C transition and does not really need to wait on a Rust port).

Declarative Session Management

Another interesting, and broader, thread is the imperative nature of WirePlumber scripts – that is, policy decisions and associated action are often interwoven. It might be helpful to be able to make a clearer split where all policy decisions are first run, and then decisions are translated into actions at one go.

There are some historical choices that make this hard – for example, changing the profile of a device might create and destroy nodes, which makes it hard to be able to make decisions that are independent of the action. There were some ideas around redoing the profile concept such that all nodes are always exposed, but nodes could get a new state to signal availability (and profiles that would allow availability to change). That might make a declarative system possible to implement.

We also discussed the possibility of a “transaction” system. Something that would allow a client to submit a set of objects (think links between nodes), and then “commit” that transaction. This would also help reduce the number of roundtrips between PipeWire and WirePlumber, and generally help performance.

Bluetooth

Being colocated with the BlueZ face-to-face meeting, we had representation from the BlueZ community, so we were able to dive into a number of topics related to Bluetooth, primarily LE Audio.

The first topic was Auracast, the LE Audio system for broadcast audio, allowing listeners to tune into public broadcasts in a space, or to have a device stream audio to multiple headsets concurrently for shared listening. George had a demo system showing an implementation of Auracast with PipeWire, WirePlumber and BlueZ.

We had some discussion of where this feature should live, and the consensus was that we would probably want a separate daemon to manage Auracast settings and loading up the appropriate nodes (either for receiving or sending) based on users’ preferences.

This led to a more general discussion about the current split of the Bluetooth implementation in PipeWire being SPA modules, which include streaming and some policy, and a lot more policy living inside WirePlumber. We could, and likely should, move all of this into higher level PipeWire modules instead, which could make these easier to work with overall.

There was also a discussion about the complexities of LE Audio, and the state of the current user experience with actual devices:

• Device interop is not always great, as the spec is new, the BlueZ implementation is still being completed, and device implementations seem of variable quality
• Reliable pairing/feature detection is hard, partly due to how BlueZ exposes the ability to talk to devices in Bluetooth Classic or Bluetooth LE modes
• Pairing left/right pairs currently needs individual pairing, which does not seem to be needed by other implementations (Android for example)
• Inter-device synchronisation might need some work as well

While there is much work to be done here, the pieces are coming together for first-class LE Audio support on Linux-based systems.

Audio analytics

We also spoke about “analytics” – using local neural networks to implement things like text-to-speech, speech-to-text, language translation, or other forms of processing.

These pose an interesting problem, because they look like a standard-ish audio stream on one side, but are effectively a sparse stream on the other side if we are talking about text. Even conversion between languages does not look like a standard filter, because the underlying model might consume a varying amount of data before generating an output, and the input and output lengths are not tightly correlated.

While it should be possible to implement such a system with PipeWire, it is not quite clear whether we should. As the application space in this area becomes more mature, it may become clearer what the right place in the stack is for these features.

Click detection and elimination

We spoke about detecting and eliminating clicks at the stop or start of a stream.

If an application is playing back audio, and suddenly stops (i.e. feeds silence, or just nothing), then the sudden drop in the signal might cause a click to be output. If you think of the corresponding waveform as representing the physical displacement of the speaker, then the drop to zero is like a sudden brake to a halt, which isn’t possible, and manifests as a jolt that you hear as a clicky noise. The same analogy holds for resuming from a pause, but in the opposite direction.

The solution is usually to smooth out the end of the sound by fading out, but most applications do not do this, so this problem manifests quite clearly for most browser or application streams if you listen closely.

Wim described a number of experiments he has done for detecting such abrupt changes in audioconvert, but he was not happy with the results. We discussed some of these approaches, and what might work as acceptable tradeoffs to capture the most common cases while still trying to respect the integrity of the signal being sent by the application.

(sorry about the vagueness here, I missed taking more detailed notes)

Miscellanea

The rest of the discussion covered disparate topics that I don’t have long form notes on:

• Hardware profiles: Shipping hardware-specific configuration for PipeWire and WirePlumber is hard. We discussed some approaches using context properties and conditions, but this is an area that needs more work.

• Data loop management: PipeWire allows splitting work across data loops so different nodes in a graph can be assigned to different threads. This is currently an all-or-nothing system, where either all nodes go to a single data loop, or every node must be manually assigned a specific data loop. There was some desire to have the ability for there to be a default data loop to make the manual management less cumbersome.

• ACP -> UCM: PipeWire inherits the ALSA card profile configuration from PulseAudio, which has been helpful in making the migration path smoother on most hardware. There was always some desire to have a single configuration system (probably ALSA UCM) for all hardware, but this likely needs some work on what we can express in UCM configuration, but we also need to clean up how we translate our UCM handling code (George has an RFC for this).

Thanks

That’s it, thank you for reading if you made it this far, and a shout out to George, Mark, and others organising the event!

It was great to see continued interest and so much exciting work that is yet to come. I hope to see more of the community in the next edition of the hackfest.

(these notes are being posted in two parts to make the length more manageable, part 2 is here)

The PipeWire community organised a hackfest in Nice, France, colocated with Embedded Recipes, the GStreamer hackfest, and a number of other events.

In attendance were members of the upstream community, as well as folks interested in PipeWire from Collabora, Red Hat, Qualcomm, Stream Unlimited, Texas Instruments, and Valve. In some cases these were the same person wearing upstream and professional hats, as some of us often do! :)

It was two days of fruitful and deep technical discussions, and lovely evenings hanging out in the Côte d’Azur. Shout out to George Kiagiadakis and Mark Filion for putting this together!

The topics were disparate and can be somewhat esoteric for folks who are not familiar with the Linux audio space. I will try to strike a balance between providing context and summarising the finer details we discussed. Please feel free to write in if I missed or can expand on anything.

Multistream nodes

A recurring topic for the last couple of years has been supporting multistream nodes. The PipeWire API currently offers a pw_stream interface that can offer a node with single input or output (closer to the PulseAudio API), and the pw_filter interface that provides a lower-level freeform API to individually manage ports on a node (closer to the JACK API).

The stream API while convenient, can be a bit unwieldy for realising concepts such as loopbacks and filters, because each set of inputs and outputs needs to be implemented as an individual node. If you’ve ever loaded the loopback module, for example, you would have noticed that there are two nodes created for each instance.

Wim has created a version of the API that allows a node to provide multiple streams, which allows us to keep the conveniences of the stream API, but more easily express ideas like the loopbacks, filters, etc. Each stream is effectively a group of ports on the node, and nodes can have an arbitrary number of input and output streams.

The code on the PipeWire side is ready. The primary idea is there will be a PortConfig param per stream, and this is where the format of the stream, and other metadata expressed on port groups (which is essentially what a stream is) will live.

We discussed what is needed in WirePlumber to make sure the linking logic adapts to this concept, and Julian will be implementing that in the coming weeks.

Settings

PipeWire has a generic metadata system based on the JACK API that is used for storing metadata (allowing you to attach a key/type/value, optionally attached to an object). This is also used by WirePlumber to provide its settings system (see wpctl settings), along with some key features such as a schema and persistence.

We discussed that it might be nicer to have the concept of settings as a first-class citizen, and possibly even standardise some settings for desktop wide usage (such as common processing elements). There was consensus that:

• A new settings interface (instead of extending metadata) would make sense
• The API should be asynchronous, and can fail
• A schema for valid settings and their types could be exposed as a well-known metadata key
• Implementors of the interface would perform validation

Security

We spoke about the current state of security for applications using PipeWire. For context, PipeWire has a fine-grained permissions model where each client can have selective access to what objects are visible to it, and what actions it may perform. There is also a less granular system, where a “manager” application can connect to the manager socket for full access. We broadly think about restricted security for sandboxed applications (primarily Flatpak).

One scenario is sandboxed PulseAudio applications getting full access via the pipewire-pulse server on the host. The discussion on this concluded that there is a way for pipewire-pulse to forward enough security-related information from sandboxed applications for us to apply sandbox restrictions to them, and we need to make that system work.

There was a discussion that it might be reasonable for our default policies to apply for all applications connecting to the regular PipeWire socket to be restricted (this does not prevent malicious applications from accessing the manager socket, but helps applications not do bad things erroneously).

This might be disruptive to introduce as a default change, so we might implement it via an opt-in setting so that there can be some broader testing and refinement of default permissions before flipping the switch for all users.

There are a number of mechanisms related to how security context properties are relayed, and how those properties are used by WirePlumber to determine permissions. We need to document and verify the expected behaviour here.

Flatpak and Portals

Relatedly there was a discussion about how things should fit in with Flatpak, and Sebastian Wick from the Flatpak team joined us briefly on the second day.

There was some discussion of making sure the PulseAudio socket is provided to the sandbox in a similar way to the PipeWire socket, such that some additional security properties can be assigned from the host in a way that the sandboxed client cannot override.

We agreed that we needed the ability for applications to specify with some granularity what permissions they require (via portals), and for us to grant only that (with user intervention, if needed). Broadly this is:

• Playback (optionally enumeration of sinks)
• Capture (optionally enumeration of sources)
• Default visibility of only the application’s own nodes

We also spoke about how we might want to associate PipeWire objects with applications. With Flatpak moving to using a cgroup for each application, this should become easier. We may also want to be able to have a way to associate a stream with a specific window (to, for example, share a window and its audio), which should be possible.

It was also noted that for some classes of applications, we may want a way for users to allow some of these permissions at install time (for example, a remote desktop application asking permission on every start can be annoying). This is already possible with Flatpak manifests (which are static, but we might need to add some more options here), and there is a potential entitlement system being discussed (for server-driven overrides to be distributed for malicious applications, for example).

Encapsulation and Collections

One topic that came up last year is the ability to encapsulate a group of nodes such that they appear as a single node to other applications in the system. This could be useful for:

• Collapsing all the output from an application so it appears to be providing a single stream
• Grouping all the filters for a sink or source node, and making it appear as a single node with all the processing hidden away

One piece to making such a system possible is to have a first-class notion of this group. Julian has an implementation of such an entity, called a “collection”. This is currently implemented on top of PipeWire metadata, but we agree that this is likely worth having an explicit PipeWire interface for.

Once that is in place, we discussed the possibility of having a smarter “proxy” node that can act as the interface that translates from the “outside” of the encapsulated region to the “inside”, so that format selection, volume changes, etc. can properly be proxied to the underlying device, for example.

Tooling improvements

It was noted that the tools we have (such as pw-top and pw-dot) can make it hard to get at some information, such as negotiated formats, rates, etc. They can also be “noisy” when we have a large number of filters and loopbacks.

While we did not have a concrete plan to tackle this, some of us have been playing with LLM-based tooling to generate some helper code for this sort of thing. At least my attempts have been too sloppy to share as yet, but it should be possible to get something useful with a structured approach.

That’s it for now. Watch this space for part 2!

It should be surprising to absolutely nobody that the Linux audio stack is often the subject of varying levels of negative feedback, ranging from drive-by meme snark to apoplectic rage^[1].

A lot of what computers are used for today involves audiovisual media in some form or the other, and having that not work can throw a wrench in just going about our day. So it is completely understandable for a person to get frustrated when audio on their device doesn’t work (or maybe worse, stops working for no perceivable reason).

It is also then completely understandable for this person to turn up on Matrix/IRC/Gitlab and make their displeasure known to us in the PipeWire (and previously PulseAudio) community. After all, we’re the maintainers of the part of the audio stack most visible to you.

To add to this, we have two and a half decades’ worth of history in building the modern Linux desktop audio stack, which means there are historical artifacts in the stack (OSS -> ALSA -> ESD/aRTs -> PulseAudio/JACK -> PipeWire). And a lot of historical animus that apparently still needs venting.

In large centralised organisations, there is a support function whose (thankless) job it is to absorb some of that impact before passing it on to the people who are responsible for fixing the problem. In the F/OSS community, sometimes we’re lucky to have folks who step up to help users and triage issues. Usually though, it’s just maintainers managing this.

This has a number of … interesting … impacts for those of us who work in the space. For me this includes:

1. Developing thick skin
2. Trying to maintain equanimity while being screamed at
3. Knowing to step away from the keyboard when that doesn’t work
4. Repeated reminders that things do work for millions of users every day

So while the causes for the animosity are often sympathetic, this is not a recipe for a healthy community. I try to be judicious while invoking the fd.o Code of Conduct, but thick skin or not, abusive behaviour only results in a toxic community, so there are limits to that.

While I paint a picture of doom and gloom, most recent user feedback and issue reporting in the PipeWire community has been refreshingly positive. Even the trigger for this post is an issue from an extremely belligerent user (who I do sympathise with), who was quickly supplanted by someone else who has been extremely courteous in the face of what is definitely a frustrating experience.

So if I had to ask something of you, dear reader – the next time you’re angry with the maintainers of some free software you depend on, please get some of the venting out of your system in private (tell your friends how terrible we are, or go for a walk maybe), so we can have a reasonable conversation and make things better.

Thank you for reading!